Intermittent :closed message caused by race condition

[elbow-jason]

When running tests I am intermittently getting this error:

12:06:12.384 [error] Process #PID<0.1286.0> raised an exception
** (CaseClauseError) no case clause matching: <payload_here>
    (cowboy) /Users/elbow-jason/my_app/deps/cowboy/src/cowboy_protocol.erl:344: :cowboy_protocol.parse_hd_value/9
12:06:12.389 [error] Ranch protocol #PID<0.1286.0> (:cowboy_protocol) of listener #Reference<0.1060090579.823656453.233967> terminated
** (exit) an exception was raised:
    ** (CaseClauseError) no case clause matching: <payload_here>
        (cowboy) /Users/elbow-jason/my_app/deps/cowboy/src/cowboy_protocol.erl:344: :cowboy_protocol.parse_hd_value/9

with a failing test that says:

  1) test client can post! and handle the response (MyApp.NotifierClientTest)
     test/my_app/notifier_client_test.exs:12
     ** (HTTPoison.Error) :closed
     code: assert ^payload = NotifierClient.post!(endpoint_url(bypass.port), %{"hello" => "world"})
     stacktrace:
       (httpoison) lib/httpoison.ex:66: HTTPoison.request!/5
       (my_app) lib/my_app/notifier_client.ex:10: MyApp.NotifierClient.post!/2
       test/my_app/notifier_client_test.exs:18: (test)

here is the source:

  setup do
    bypass = Bypass.open
    {:ok, bypass: bypass}
  end

  test "client can post! and handle the response", %{bypass: bypass} do
    payload = %{"errors" => [%{"code" => 88, "message" => "Rate limit exceeded"}]}
    Bypass.expect_once bypass, "POST", "/", fn conn ->
      body = Poison.encode!(payload)
      Plug.Conn.resp(conn, 200, body)
    end
    assert ^payload = NotifierClient.post!(endpoint_url(bypass.port), %{"hello" => "world"})
  end

  defp endpoint_url(port), do: "http://localhost:#{port}/"

If I put a timer.sleep(100) in the setup block it stops happening.

Any suggestions/ideas for a solution?

[elbow-jason]

I was dead wrong. Sorry. I was not encoding a binary correctly and it was failing intermittently. :-/